302 DISTRIBUTION OF PLANTS. [SECT.XXVII. 



forests, while, with their decreasing energy in the higher 

 latitudes, vegetation becomes less and less vigorous. 



By far the greater part of the hundred and ten thousand 

 known species of plants are indigenous in Equinoctial Ame- 

 rica. Europe contains about half the number ; Asia, with its 

 islands, somewhat less than Europe ; New Holland, with the 

 islands in the Pacific, still less ; and in Africa there are 

 fewer vegetable productions than in any part of the globe of 

 equal extent. Very few social plants, such as grasses and 

 heaths, that cover large tracts of land, are to be found between 

 the tropics, except on the sea-coasts and elevated plains : 

 some exceptions to this, however, are to be met with in the 

 jungles of the Deccan, Khandish, &c. In the equatorial 

 regions, where the heat is always great, the distribution of 

 plants depends upon the mean annual temperature ; whereas 

 in temperate zones the distribution is regulated in some de- 

 gree by the summer heat. Some plants require a gentle 

 warmth of long continuance, others flourish most where the 

 extremes of heat and cold are greater. The range of wheat 

 is very great: it may be cultivated as far north as the 60th 

 degree of latitude, but in the torrid zone it will seldom form 

 an ear below an elevation of 4500 feet above the level of the 

 sea, from exuberance of vegetation ; nor will it ripen above 

 the height of 10,800 feet, though much depends upon local 

 circumstances. Colonel Sykes states that in the Deccan 

 wheat thrives 1800 feet above the level of the sea. The best 

 wines are produced between the 30th and 45th degrees of 

 north latitude. With regard to the vegetable kingdom, 

 elevation is equivalent to latitude, as far as temperature is 

 concerned. In ascending the mountains of the torrid zone, 

 the richness of the tropical vegetation diminishes with the 

 height ; a succession of plants similar to, though not identical 

 with, those found in latitudes of corresponding mean tempe- 

 rature takes place ; the lofty forests by degrees lose their 

 splendour, stunted shrubs succeed, till at last the progress of 

 the lichen is checked by eternal snow. On the volcano of 



